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Gas sensor based on dynamic thermal conductivity and molecular velocityRelated Patent Categories: Measuring And Testing, Gas Analysis, By Thermal Property, ThermoconductivityGas sensor based on dynamic thermal conductivity and molecular velocity description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20070169541, Gas sensor based on dynamic thermal conductivity and molecular velocity. Brief Patent Description - Full Patent Description - Patent Application Claims
BACKGROUND [0001] 1. Field of Invention [0002] This invention relates generally to gas sensors and methods of detecting gases. [0003] 2. Related Art [0004] Gas detection methods based on the thermal conductivity transport property have been widely used because such methods do not show significant selectivity toward a specific gas. Thermal conductivity is defined as the rate at which heat flows through an area of a body or material. Since thermal conductivity is a bulk property of gases, methods based on this property are considered to be near universal or non-specific gas detection methods. [0005] Typically, the thermal conductivity of a gas is measured by heating a resistance element such as a hot wire filament or a thermistor, and contacting the heated element with a gas sample. The temperature of the resistance element is determined by the thermal conductivity of the gas sample, with a change in temperature reflected as a change in resistance of the resistance element. In this method, the resistance element behaves as a thermal conductivity sensing element. [0006] Certain gases, such as helium and hydrogen, have thermal conductivities that are much greater than the thermal conductivity of air, while other gases, such as nitrogen, argon, carbon dioxide, carbon monoxide, ammonia and nitrogen have thermal conductivities that are less than or similar to that of air. A detector such as a gas chromatograph, which measures thermal conductivity of a gas typically uses a carrier gas of high thermal conductivity to detect a specimen gas of low thermal conductivity, or a carrier gas of low thermal conductivity to detect a specimen gas of high thermal conductivity. For example, helium is used as a carrier gas for nitrogen detection, and nitrogen or argon is used as a carrier gas for hydrogen detection. In other thermal conductivity detector applications, specific gas analysis based on thermal conductivity has been limited to either binary gas mixtures of known gas species, or hydrogen gas in a mix of gases having similar thermal conductivities significantly different from hydrogen. Because these methods measure only a single bulk parameter of a gas sample, different gas mixtures having similar thermal conductivities cannot be distinguished. SUMMARY [0007] The present invention provides a device and a method for gas analysis in which three parameters--velocity, thermal conductivity and temperature--can be measured for a single gas sample. By determining three parameters in a single sample of gas, different gas mixtures having similar thermal conductivities can be distinguished, and gas mixtures having more components than binary compositions can be analyzed. In particular embodiments, the velocity, thermal conductivity and temperature of a single gas sample can be determined under high pressure and high temperature conditions. Further, the detection of hydrogen gas can be accomplished with less interference from background gases than existing hydrogen gas analyzers. [0008] A device in accordance with the present invention is a gas sensor that includes: a) a sample chamber for holding a gas sample; b) a sealable vacuum port in fluid communication with the sample chamber, for evacuating the sample chamber; c) a sealable inlet port in fluid communication with the sample chamber, for introducing the gas sample into the evacuated sample chamber; and d) a thermal conductivity sensing element at least partly disposed within the sample chamber, for measuring the thermal conductivity of the gas sample. The gas sample can comprise one or more gaseous components. In the device, the inlet port is positioned relative to the thermal conductivity sensing element such that movement of the gas sample from the inlet port to the sensing element takes an interval of time. This time interval is a measure of the velocity of the gas sample. Thus, movement of the gas sample from the inlet port to the sensing element provides a measure of gas sample speed. Unlike other gas sensors, the device of the present invention can be fabricated without wetted or exposed parts, which cannot withstand high temperature and high pressure conditions. As such, the device can be used over a broader range of temperatures and pressures compared to other gas sensors. [0009] The present invention also provides a method of analyzing a gas. The method comprises: a) creating a vacuum in a sample chamber of a gas sensor assembly; b) releasing a gas sample into the evacuated sample chamber; c) determining the speed of the gas sample; and d) determining the thermal conductivity of the gas sample. [0010] The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present invention. BRIEF DESCRIPTION OF DRAWINGS [0011] FIG. 1 is a schematic drawing of a gas sensor; [0012] FIG. 2 is a schematic drawing of a filament and pressure seal; [0013] FIG. 3 is a diagrammatic representation of a single measurement cycle; [0014] FIG. 4 is a block diagram of a gas sensor assembly; [0015] FIG. 5 is a drawing of a Wheatstone Bridge circuit incorporating a thermal conductivity sensing filament and a reference filament; [0016] FIG. 6 is a graph of bridge delta voltage verse gas thermal conductivity; [0017] FIG. 7 is a graph of sensor resistance verse absolute temperature; [0018] FIG. 8 is a graph showing gas speed of various gas samples; and [0019] FIG. 9 is a graph showing voltage across a metal filament as a function of time. DETAILED DESCRIPTION [0020] A schematic drawing of a sensor according to the present invention is shown in FIG. 1. The sensor comprises a sample vessel 2 having a sample chamber 4 in fluid communication with an inlet port 6 and a vacuum port 8. A thermal conductivity sensing element 10 is at least partially disposed in the sample chamber and is connected to a controller 12. A sample control valve 14, for controlling gas sample entry into the sample chamber, is connected to the inlet port 6. A vacuum control valve 16, for opening and closing the vacuum port 8, is connected at one end to the vacuum port 8 and at the other end to a source of vacuum 18. In this embodiment, the entire assembly, except for the vacuum source, is placed in a housing or sensor block 20, which can be fabricated from materials such as stainless steel, copper, brass, carbon steel and the like. However, as will be apparent to those skilled in the art, various parts of the gas sensor assembly can be separately located. For example, in other embodiments, the sample control valve, the vacuum control valve and/or the controller can be located externally to the sensor block. In FIG. 1, the gas sensor is shown connected to the flow 22 of a gas production process. Continue reading about Gas sensor based on dynamic thermal conductivity and molecular velocity... Full patent description for Gas sensor based on dynamic thermal conductivity and molecular velocity Brief Patent Description - Full Patent Description - Patent Application Claims Click on the above for other options relating to this Gas sensor based on dynamic thermal conductivity and molecular velocity patent application. ###  How KEYWORD MONITOR works... a FREE service from FreshPatents1. Sign up (takes 30 seconds). 2. Fill in the keywords to be monitored. 3. Each week you receive an email with patent applications related to your keywords. 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